Due to the potential gains in spectral efficiency that can be achieved through simultaneous uplink (UL) and downlink (DL) communication within the entire frequency band, in-band full-duplex, also known as full-duplex (FD), is a promising candidate technology for next generation wireless communication systems. Commonly used half-duplex (HD) systems, such as time division duplex (TDD) or frequency division duplex (FDD) systems, employ orthogonal time resources or orthogonal frequency resources, respectively. A full-duplex BS can improve the spectral efficiency because it uses the same time and frequency resources for uplink and downlink simultaneously. FIG. 1 shows an exemplary communication system 100 including two full-duplex base stations BS0 and BS1 simultaneously using the same time and frequency resources for uplink and downlink communication with user equipments 101a, b and user equipments 101c, d, respectively.
Thus, enabling full-duplex base stations can potentially double the spectral efficiency at the cell. However, the simultaneous transmission and reception in full-duplex base stations gives rise to a new interference scenario, e.g., inter-cell interference between neighboring co-channel base stations and self-interference at a single base station. Examples of inter-cell interference and self-interference are shown in FIG. 1 for the case of a communication system 100 with two BSs and in FIG. 2 for a more general case of a communication system 200 with seven BSs.
In currently deployed wireless systems, co-channel base stations are typically synchronized such that all cells use the same uplink-downlink configuration with the transmission direction (either uplink or downlink) in all cells being time aligned. The main reason for a synchronous operation is that the usage of opposite transmission directions in neighboring co-channel cells would result in strong base station to base station interference (Z. Shen, A. Khoryaev, E. Eriksson and X. Pan, “Dynamic uplink-downlink configuration and interference management in TD-LTE”, IEEE Communications Magazine, November 2012). Consequently, inter-cell interference and the self-interference can be avoided in half-duplex networks by using an appropriate time and/or frequency slot assignment. This is not the case in full-duplex BS deployments, because simultaneous co-channel uplink and downlink transmissions in all base stations are the essence of the full-duplex operation. In consequence, the self-interference and the base station to base station inter-cell interference must be explicitly addressed in order to enable full-duplex networks and leverage the potential increase of spectral efficiency (S. Goyal, P. Liu, S. S Panwar, R. A. DiFazio, R. Yang, and E. Bala “Full duplex cellular systems: Will doubling interference prevent doubling capacity?”, IEEE Communications Magazine, May 2015 and Y. S. Choi and H. Shirani-Mehr “Simultaneous Transmission and Reception: Algorithm, Design and System Level Performance”, IEEE Transactions on Wireless Communications, December 2013).
Current solutions for interference mitigation in half-duplex systems have disadvantages when applied to the full-duplex scenario. For example, if one applies an Almost Blank Subframe (ABS) solution like the one proposed in LTE (K. I. Pedersen, Y. Wang, B. Soret and F. Frederiksen, “eICIC functionality and performance for LTE HetNet co-channel deployments”, in Proc. IEEE Vehicular Technology Conference Fall, 2012), where a set of the interfering nodes remains silent during a period of time, the full-duplex configuration ends up being reverted (completely or partially) to a half-duplex configuration hence losing some or all of the full-duplex gain. Similarly, solutions like frequency reuse or fractional frequency reuse as disclosed, for instance, in T. Novlan, J. G. Andrews, I. Sohn, R. K. Ganti and A. Ghosh, “Comparison of fractional frequency reuse approaches in the OFDMA cellular downlink”, in Proc. IEEE Global Telecommunication Conference, 2010, which do an orthogonal frequency assignment to regions that suffer from interference, end up reverting the full-duplex assignment (completely or partially) to a half-duplex one.
Conventional full-duplex solutions mainly consider the problem of self-interference cancellation while disregarding the problem of inter-cell interference. The self-interference received at a full-duplex base station is due to the received signal from its own transmissions, whereas the inter-cell interference is due to the received signal from the transmissions of neighboring co-channel base stations. In the case of multiple antenna full-duplex base stations, the use of large number of antennas has only been leveraged for reducing the self-interference, i.e., for achieving spatial self-interference cancellation (B. Yin, M. Studer, J. R. Cavallaro and J. Lilleberg, “Full-duplex in large-scale wireless systems”, in Proc. Asilomar Conference on Signal Systems and Computers, November 2013 and H. Q. Ngo, H. A. Surawera, M. Matthaiou and E. G. Larsson, “Multipair full-duplex relaying with massive arrays and linear processing,” available online http://arxiv.org/abs/1405.1063).
Generally, conventional spatial self-interference cancelling solutions implicitly assume that the size of the antenna array is larger than the number of users to be served in the cell plus the number of independent self-interference signals to be cancelled. This premise, however, does not usually hold for the case of inter-cell interference. This is because the number of co-channel neighboring base stations' antennas and, thus, the number of independent interfering directions to be cancelled is very high and can potentially increase with the size of the base stations arrays. Hence, extending the self-interference cancellation solutions disclosed in the above-referenced papers by Yin et al. and Ngo et al. for simultaneously solving the self-interference and the inter-cell interference problems is difficult. Indeed, the case when the number of degrees of freedom provided by the large scale multi antenna configuration is smaller than the number required for interference nulling between full-duplex base stations has not been considered. In such a case, the base station DL precoder must consider the tradeoff between serving its DL users and minimizing the self-interference and inter-cell interference.
Massive MIMO technology uses antenna arrays with the number of antenna elements being some orders of magnitude larger than current state-of-the-art MIMO technology, say 100 antennas or more (F. Rusek, D. Persson, B. K. Lau, E. G. Larsson, T. L. Marzetta, O. Edfors, and F. Tufvesson, “Scaling up MIMO: Opportunities and challenges with very large arrays”, IEEE Signal Processing Magazine, January 2013 and E. G. Larsson, F. Tufvesson, O. Edfors, and T. L. Marzetta, “Massive MIMO for Next Generation Wireless Systems”, IEEE Communications Magazine, February 2014). Reaping the benefits of massive MIMO technology requires the channels of the active users to be nearly orthogonal or very low correlated. Real channel measurements campaigns reported in Xiang Gao; Edfors, O.; Rusek, F.; Tufvesson, F., “Linear Pre-Coding Performance in Measured Very-Large MIMO Channels,” in Vehicular Technology Conference (VTC Fall), 2011 IEEE, vol., no., pp. 1-5, 5-8 Sep. 2011 and Hoydis, J.; Hoek, C.; Wild, T.; ten Brink, S., “Channel measurements for large antenna arrays,” in Wireless Communication Systems (ISWCS), 2012 International Symposium on, vol., no., pp. 811-815, 28-31 Aug. 2012 indicate that channel correlation cannot be arbitrarily reduced by increasing the number of antennas, which means that usual propagation environments offer a limited number of physical directions or degrees of freedom. Hence, the number of users that can be effectively served within a cell is limited (around 20 as reported in Hoydis et al. 2012) independently of the number of antennas at the massive MIMO base station (typically larger than 100). This leads to the concept of excess antennas. The number of excess antennas is given by the number of transmit antennas minus the number of active DL users.
The use of excess antennas in a massive MIMO base station to mitigate the inter-cell interference has been considered in J. Hoydis, K. Hosseni, S. T. Brink and M. Debbah, “Making smart use of excess antennas: Massive MIMO, small cells, and TDD”, Bell Labs Technical Journal 18(2), 5-21, 2013 in the context of HD two-tier heterogeneous networks. However, the solutions provided in Hoydis et al. 2013 hold only for the case where all of the antennas in the array are either in transmitter mode or in receiver mode. Hence, the solutions provided in Hoydis et al. 2013 apply only to a half-duplex scenario.
In the light of the above, there is a need for an improved apparatus and method for managing full-duplex communication between a base station and a plurality of user equipments allowing the base station to serve the plurality of user equipments while mitigating the inter-cell interference that the base station generates at neighboring full-duplex base stations.